Show HN: Accelerated Docker builds on your local machine with Depot (YC W23)

Hello HN! We just launched a new feature we built at Depot that accelerates Docker image builds on your local machine in a team environment, and we wanted to share some of the details with you all.

The launch blog post: https://depot.dev/blog/local-builds

Depot is a hosted container build service - we run fully managed Intel and Arm remote build machines in AWS, with large instance sizes and SSD cache disks. The machines run BuildKit, the build engine that powers Docker, so generally anything you can `docker build`, you can also `depot build`.

Most people use Depot in CI, and you could also run `depot build` from your local machine as well. That would perform the build using the remote builder, with associated fast hardware and extra fast datacenter network speeds.

But then to download the container back to your local machine, BuildKit would transfer the entire container back for every build, including base image layers, since BuildKit wasn’t aware of what layers already existed on your device.

The new release fixes this! To make it work, we replaced the BuildKit `--load` by making the Depot CLI itself serve the Docker registry API on a local port, then asking Docker to pull the image from that localhost registry. The CLI in turn intercepts the requests for layers and fetches them directly using BuildKit’s content API.

This means Docker only asks for the layers it needs! This actually speeds up both local builds, where you only need to download changed layers, as well as CI where it can skip building an expensive tarball of the whole image every time!

We ran into one major obstacle when first testing: the machine running the Docker daemon might not be the same machine running the `depot build` command. Notably, CircleCI has a remote Docker daemon, where asking it to pull from localhost does not reach the CLI’s temporary registry.

For this, we built a "helper" container that the CLI launches to run the HTTP server portion of the temporary registry - since it’s launched as a container, it does run on the same machine as the Docker daemon, and localhost is reachable. The Depot CLI then communicates with the helper container over stdio, receiving requests for layers and sending their contents back using a custom simple transport protocol.

This makes everything very efficient! One cool part about the remote build machines: you can share cache with anyone on your team who has access to the same project. This means that if your teammate already built all or part of the container, your build just reuses the result. This means that, in addition to using the fast remote builders instead of your local device, you can actually have cache hits on code you haven’t personally built yet.

We’d love for you to check it out, and are happy to answer any questions you have about technical details!

https://depot.dev/docs/guides/local-development